An accurate measurement of physical parameters of a device (for example, optical parameters of an optical device) is crucial in a procedure of device measurement and fabrication. Contact measuring methods require polishing processing on a surface of a unit to be measured, which causes abrasion to the unit, thus non-contact measuring methods are usually adopted at present.
An interferometric measurement plays an important role in the non-contact measuring methods, and a key point thereof is to analyze and process an interference fringe pattern (for example, a Newton's rings fringe pattern) generated by performing the interferometric measurement on the unit to be measured, so as to obtain various physical parameters, such as curvature radius, vertex position, wavelength of incident light, refractive index of media, deformation, displacement, film thickness or the like, of the unit.
A simplified method for processing the Newton's rings fringe pattern (a basic interference fringe pattern) is a ring-counting calculating method, which measures the interference fringe pattern by using a reading microscope, obtains diameter values of two orders of dark fringes, and calculates a physical parameter of the unit to be measured on the basis of wavelength of incident light and the diameter values. Generally speaking, for the sake of measurement accuracy, a diameter of a high-order ring, e.g., up to the 40th-order ring beginning from the 0-order center-ring, needs to be measured. The above-described ring-counting calculating method is simple to implement and has a low cost. However, it is easy for an observer to make mistakes due to visual fatigue when counting the rings, and the automation degree is low for this method. Further, since a field range of the reading microscope is relatively small, it is not possible for the observer to see the entire interference fringe pattern, i.e., intuitiveness thereof is poor.
More commonly, a typical method for processing the interference fringe pattern is a fringe centerline method, and a processing flow thereof includes: 1) denoising the interference fringe pattern; 2) performing binarization and thinning on the denoised interference fringe pattern; 3) deriving a radius and a center of the interference fringe by acquiring coordinate values of points on the thinned fringes; and 4) calculating a physical parameter of the unit to be measured, by using radius of two orders of bright or dark fringes and the wavelength of incident light. It can be seen that, as compared with the ring-counting calculating method, the fringe centerline method has a higher automation degree and a better intuitiveness.